Just 9% were jailed between July and September – down from 11% over the same three months last year,

Nine out of 10 youths caught with blades walk free – despite David Cameron’s election pledge to lock them all up.

Just 9% were jailed between July and September – down from 11% over the same three months last year, according to Ministry of Justice figures.

The proportion of adults jailed for knife crime has also slumped from 24% to just 22%.

The figures released today make a mockery of the PM’s tough stance in the run up to the 2010 election.

Mr Cameron said at the time: “We have to send a clear message that carrying a knife on our streets is completely inexcusable and unacceptable in a civilised society.

“So we’re proposing that anyone convicted of knife crime should expect to go to jail.”

Mr Cameron’s remarks came after the Mirror’s Stop Knives, Save Lives campaign was launched in the wake of the stabbing death of Ben Kinsella, 16, in North London in 2008.

The Mirror's campaign

The PM’s Justice Secretary, Chris Grayling, last week introduced a new offence of aggravated knife crime with the aim of jailing more offenders, saying he was “not happy” with the number being let off with cautions.

“If you’re wandering round carrying a knife in a threatening way, you will go to jail,” he said.